1. Technical Field
The invention relates to microwave radiometry, and in particular to an end-to-end microwave receiver, such as a water vapor radiometer, formed of monolithic microwave integrated circuits on a single miniature integrated hybrid package system.
2. Background Art
Microwave components used in communication and radiometry typically consist of microwave circuits, microwave resonators and the like connected by microwave waveguides, resembling a plumbing maze. A complete end-to-end system, such as a microwave water vapor radiometer, includes a receiver antenna at the input end, all of the required frequency down-conversion circuitry and an analog-to-digital function at the output end. The amount of hardware required occupies a volume typically on the order of an average suitcase. Due to their large size, such systems are susceptible to errors due to temperature fluctuations in different parts of the system, although the analog-to-digital function at the output end produces a digital output signal which is impervious to temperature fluctuations. Due to their large weight, such systems are not readily transportable and are subject to breakdown due to vibration. Moreover, the cost of materials and the cost of manufacturing such systems is relatively high due to their type and size.
A more serious problem concerns microwave radiometry of extremely small signals requiring exceptional instrument sensitivity. For example, microwave radiometry was used to establish the uniformity of background radiation throughout space attributed to residual noise from the primordial "big bang", and is currently used to sense the background radiation at the water absorption lines around 20 GHz and 30 GHz to measure atmospheric moisture concentration in studies relating to aircraft wing ice formation and global warming. In all such cases, the radiation levels to be measured are extremely small. Temperature-induced fluctuations in the response characteristics of the microwave circuits in a microwave radiometer introduce performance limiting errors into any measurements which the radiometer makes. Such radiation is typically present in extremely small amounts, comparable in magnitude to fluctuations induced by small temperature variations in the microwave circuit elements of the radiometer. Accordingly, great attention is given to improving techniques for maintaining constant uniform temperature throughout the relatively large assembly of microwave components comprising the radiometer. Of course, the difficulty of maintaining a constant uniform temperature is proportional to the size of the radiometer. As a result, there has always been a fundamental limitation in the accuracy with which such devices could measure microwave radiation. This fundamental limitation arises from noise in the microwave circuit components attributable to small internal temperature fluctuations which are unavoidable in the typical bulky microwave radiometer.
Microwave systems such as radiometers are bulky in part because they consist of many hybrid circuits connected together. Each hybrid circuit consists of several integrated circuits or discrete components mounted in a single package. The waveguide hardware required to interconnect all of the hybrid circuits in the system is of considerable bulk. Heretofore, it has been feasible to implement only a limited function in each hybrid circuit. For example, a given hybrid circuit may be a mixer, or an amplifier or a local oscillator. Many such hybrid circuits are required to make a complete microwave system such as a microwave radiometer.
There is a trade-off in increasing the number of functions or circuits in a single hybrid package. On the one hand, it is less costly to manufacture a given set of functions or circuits if they are all contained in a single hybrid package, instead of being broken out into separate hybrid packages which must be interconnected. On the other hand, diagnosis and modular replacement of individual functions within the set which have failed is practical only if the functions are in separate packages. If all functions are contained in a single hybrid package, the failure of one function or component therein renders the entire device useless unless costly disassembly efforts are made. The system becomes less modular as more functions are implemented on a single hybrid circuit, a significant tradeoff. Such a tradeoff has seemed unavoidable in the prior art.
Moreover, increasing the capability of a single hybrid circuit requires that more integrated circuits or discrete components be connected together in a single hybrid circuit package. This in turn dramatically increases the difficulty and cost of manufacturing. During manufacture, the lid of the circuit package is removed and all soldering of connections or wire bonding must be done within the space contained within the four side walls of the circuit package. The limitations of working space and poor access impose practical constraints on the number of integrated circuits or components that can be included in a single hybrid circuit.
It is an object of the invention to radically reduce the size of an entire microwave device, such as a microwave radiometer, so as to greatly improve the tendency of a heat sink to maintain a virtually constant uniform temperature throughout the device.
It is a related object of the invention to provide an entire microwave device, such as a microwave radiometer, on a single miniature hybrid microwave circuit substrate supporting many monolithic microwave integrated circuits.
It is another object of the invention to provide a miniature hybrid microwave circuit substrate with easy three-dimensional access to all wire connections.
It is a still further object of the invention to provide a miniature hybrid microwave circuit substrate in which modular sections thereof may be removed without interference from other connected devices or protruding connector pins.